Page-wide array (PWA) inkjet printers have a stationary print head with thousands of ink nozzles, often at resolutions as high as 1600 or 3200 dots per inch (dpi). Since the print head is stationary during printing, one cannot correct or compensate for an inoperative nozzle without first detecting the inoperative nozzle, and then taking measures to correct or compensate for the inoperative nozzle (e.g., by performing a nozzle flush or other print-head maintenance procedure). Therefore, it is critical that inoperative nozzles be detected so that corrective measures (e.g., dither patterns and/or other nozzle mapping or maintenance procedures) can be employed. These corrective measures, in general, should not be done proactively unless a nozzle is inoperative, as other print artifacts and/or print quality (PQ) reductions and/or print head life reductions could occur.
Some PWA inkjet printers can have a very high resolution optical sensor (e.g., a scanning head optical sensor) to monitor the output of the printer. Such optical sensors can identify inoperative nozzles. However, on lower cost, compact printers, the cost and size associated with such a high resolution optical sensor can be prohibitive. Many such lower cost printers (e.g., “all-in-one” type printers) have a low-cost flat bed or sheet-fed scanner of a lower resolution (e.g., 300 or 600 dpi) included as an integral part of the product.
FIG. 1 illustrates the above-described problem. An image comprising a continuous array of pixels 101, containing an error in the form of a column of missing pixels 103, is placed on a scanner. Scan bar 100, comprising scan sensors 102, has a resolution lower than that of the pixels 101 in the imaging device from which the image was produced. As a result, missing pixels 103 cannot be “seen” or detected individually by scan sensor 102a because scan sensor 102a detects adjacent pixel 101a. Similarly, scan sensor 102b cannot detect missing pixels 103 because scan sensor 102b detects adjacent pixel 101b. Accordingly, as shown, missing pixels 103 cannot be detected individually by either scan sensor 102a or adjacent scan sensor 102b. As scan bar 100 travels down the page from position P1 to P2 to P3, missing pixel 103s are not detected by any single scan sensor, irrespective of the position of scan bar 100.
Accordingly, the typical resolutions of low-cost scanners (e.g., 300 or 600 dpi) are generally too low to directly identify inoperative nozzles in an image produced by a PWA print head.